1. Field of the Invention
This invention relates to a method and apparatus for detecting position deviation between grids of an assembled electron gun.
2. Description of the Related Art
An electron gun, arranged in a cathode ray tube used in a television receiver for radiating an electron beam to a phosphor surface, is made up of a cathode for emitting an electron beam and a plurality of grids making up an electrode.
This electron gun is assembled so that plural grids are arrayed on the same axial line and a cathode is arranged in the lowermost one of these plural grids (first grid).
The electron gun is configured so that an electron beam radiated from the cathode is passed through the grids so as to be controlled, accelerated and converged to illuminate a predetermined phosphor on the phosphor surface to emit light from this phosphor.
If the electron gun is a single type electron gun (one gun three beam type), the first grid is made up of three bottomed cylindrically-shaped in-line single grids. In the inside of the single grids, a cathode for emitting light from a red phosphor, another cathode for emitting light from a green phosphor and yet another cathode for emitting light from a blue phosphor are arranged independently of one another.
At a mid portion of the bottom surface of each of three single grids making up the first grid is formed an extremely small electron beam transmitting hole for transmitting the electron beam emitted from the associated cathode. In a facing surface to the first grid of another grid (second grid) arranged in adjacency to the first grid are formed three extremely small electron beam transmitting holes. The bottom surface of the first grid and the facing surface of the second grid are abutted to each other so that the electron beam transmitting holes provided in the three single grids will be coincident with those provided in the second grid.
This one-gun three-beam type electron gun transmits the electron beams emitted from each cathode through the associated electron beam transmitting holes so that the three beams intersect one another at the center of the lens constituted by plural grids and then diverge from one another so as to undergo refraction on an electro-static polarizing plate and so as to be converged on the phosphor surface.
If the grids making up the electron gun are poor in assembling accuracy, there is produced deviation in the shape or trajectory of the outgoing electron beams. In particular, since the first grid G1 and the second grid G2 are positioned so that extremely small electron beam transmitting holes 100, 101 will be in register with each other, if there is induced position deviation exceeding an allowable limit as shown in FIG. 1, the electron beam emitted from the cathode 102 is not transmitted properly through the electron beam transmitting holes, such that significant deviation is produced in the shape or the trajectory of the electron beam.
If the electron gun in which the electron beam is deviated in shape or trajectory is mounted in the cathode ray tube, the electron beams are not illuminated at a proper position on the phosphor surface with a proper shape thus deteriorating the picture quality.
Such position deviation of an electron gun is conventionally detected by a position deviation detection device 110 shown in FIG. 2. This position deviation detection device 110 shown in FIG. 2 is made up of an XY stage 111 supporting an electron gun 120 for movement in the X-axis direction and in the Y-axis direction, a fiber lighting device 112 for lighting the electron gun 120 supported on the XY stage 111 from its lateral side, and a microscope 113 by which a viewer can observe the electron gun 120 illuminated by the fiber lighting device 112 so as to view the cathode arranged in the inside of the lowermost grid through respective grids from above the uppermost grid.
The electron gun 120 is set on the setting surface of the XY stage 111 of the position deviation detection device 110 so that the center axis of the gun will be substantially perpendicular to this setting surface, and the light is directed from the fiber lighting device 112 from the lateral side for illuminating the space between the grids.
For detecting position deviation between the first grid G1 and the second grid G2, an edge of the electron beam transmitting hole 100 of the illuminated first grid G1 and an edge of the electron beam transmitting hole 101 of the second grid G2 are observed with a microscope 113 to detect the position deviation.
In the detection method employing the conventional position deviation detection device 110, in which the electron gun is illuminated from its lateral side by the electron gun 120, the volume of light that can be observed is small as compared to the volume of the illuminating light, so that the so-called light utilization efficiency is low, with the field of view being dark. Since the field of view is dark, the grid cannot be observed even with the increased magnification ratio of the microscope so that the position deviation of the electron gun 120 occasionally cannot be detected accurately.
In the conventional detection method employing the conventional position deviation detection device 110, in which the electron gun is illuminated from its lateral side by the electron gun 120, the grid surface cannot be illuminated uniformly, so that the entire grid cannot be observed evenly.
As a method for increasing the utilization efficiency of the illuminating light and evenly illuminating the light on the grid surface in order to improve the position deviation, it may be contemplated to illuminate the light from the uppermost grid of the electron gun 120 in a direction along the center axis of the grid to detect the position deviation based on the observation of the reflected light by downward perpendicular illumination. However, if the position deviation between the first grid G1 and the second grid G2 is to be detected by this downward perpendicular illumination, the edge of the electron beam transmitting hole 100 of the illuminated first grid G1 and the edge of the electron beam transmitting hole 101 of the second grid G2 cannot be observed clearly to render it difficult to detect the position deviation.
That is, the first grid G1 and the second grid g2 are formed by flat metal surfaces in which the electron beam transmitting holes 100, 101 are formed by punching, with the vicinity of the edges of the electron beam transmitting holes 100, 101 being tilted in the punching direction. Thus, the volume of the reflected light from the edges of the surfaces of the first grid G1 and the second grid G2 by the downward perpendicular illumination is extremely small as compared to the volume of the light reflected from the other portions.
The light traversing the electron beam transmitting holes 100, 101 and reflected by the electron beam emitting surface of the cathode 102 arranged in the inside of the first grid G1 undergoes irregular reflection and hence is reduced in light volume.
Thus, if it is attempted to detect the position deviation between the first grid G1 and the second grid G2 by downward perpendicular illumination, the light reflected by the vicinity of the edges of the electron beam transmitting holes 100, 101 is darkly mixed with the light reflected by the electron beam radiating surface of the cathode 102, so that the edges of the electron beam transmitting holes 100, 101 of the first grid G1 and the second grid G2 cannot be discerned clearly.